The long-range aim of this proposal is to improve our understanding of mechanisms of irregular vocal fold vibration, relevant for the prevention and treatment of voice disorders. While irregular vocal fold vibration is characteristic of many voice disorders, little is known regarding the mechanisms of irregular vibration. Mechanisms of irregular vibration will be investigated using sophisticated, quantitative descriptions of vocal fold dynamics, along the medial surface of the vocal folds, modal analysis, and computer modeling. Conventional imaging of laryngeal vibrations with endoscopy, using either stroboscopy or highspeed imaging, is limited to a superior view of the folds. Unfortunately, this view conceals the medial surface of the vocal folds during glottal closure and prohibits direct measurement of vertical vibration. Because of small size and inaccessibility, few studies have attempted to quantify the vibrations of the medial surface of the vocal folds. However, during the past five years of R29 support, Dr. Berry has collected highspeed images of coronal cross-sections of the medial surface of canine vocal folds using hemi-larynx experiments. Quantitative measurements were performed on the highspeed images, and modal analysis was conducted. The power of modal analysis was demonstrated in its ability to reduce complex vibrations to essential dyna.mics, thus revealing mechanisms of irregular vibration. Over the next five-year period, Dr. Berry proposes to augment these studies and extend their applicability to human phonation by addressing the following Specific Aims: (1) Investigate the influence of glottal geometry on the basic vibrational modes of the human larynx, using the hemi-larynx methodology developed previously by Dr. Berry; (2) Investigate the influence of the vocal tract on the basic vibrational modes of the human larynx, again using the hemi-larynx methodology; (3) Investigate the influence of the thyroarytenoid muscle on the basic vibrational modes of the vocal folds, using the heini-larynx methodology on an in vivo canine; (4) Investigate the influence of neuromuscular asymmetries on the basic vibrational modes of the vocal folds, using a computational model and highspeed images from the clinic.